Biochemical and biological assays are a primary tool utilized in many aspects of drug discovery, including but not limited to (1) fundamental research in biochemistry and biology to describe novel phenomena, (2) analysis of large numbers of compounds, (3) screening of compounds, (4) clinical tests applied during clinical trials, and (5) even diagnostic tests during administration of drugs. Many biological and biochemical assays require measurement of the response of a biological or biochemical system to different concentrations of one reagent, such as an inhibitor, an activator, a substrate, or an enzyme. Typically, discrete steps of biochemical concentration are mixed within a prescribed range. The number of concentrations measured is limited by the number of dilution steps, which are limited in practice by the time and effort required to make the discrete dilutions, by the time and effort to process the resulting individual reactions, by reagent consumption as the number of reactions increases, and more strictly by pipetting errors that limit the resolution of discrete steps.
As technology advances in drug development, miniaturization and automation are active areas of innovation, with primary drivers being decreased cost (through decreased reagent use and decreased manpower) and improved data quality (through finer process control and increased process reliability). Improvements in data quality and automation frequently convey additional advantages that permit new scientific approaches to questions. Automation, if sufficiently extensive, can include software that permits automatic work scheduling to improve efficiency or statistical process control for process improvement. Again, these improvements achieve greater reliability, use less manpower, and improve throughput.
Microfluidic systems, including labs-on-a-chip (LoCs) and micro-total analysis systems (μ-TAS), are currently being explored as an alternative to conventional approaches that use microtiter plates. The miniaturization afforded by microfluidic systems has the potential to greatly reduce the amount of reagent needed to conduct high-throughput screening. Thus far, commercial microfluidic systems have shown some promise in performing point measurements, but have not been employed to mix concentration gradients and particularly continuous gradients due to technologic limitations. In particular, several challenges remain in the design of industry-acceptable microfluidic systems. Apart from cost and manufacture related issues, many sources of such challenges relate to the fact that, in a micro-scale or sub-micro-scale environment, certain fluid characteristics such as viscosity, surface tension, shear resistance, thermal conductivity, electrical conductivity, molecular diffusivity, and the like, take on a much more dominant role than other, more easily manageable factors such as weight and gravity. In addition, controlling the signal-to-noise ratio becomes much more challenging when working with nano-scale volumes and flow rates, as certain sources of noise that typically are inconsequential in macroscopic applications now become more noticeable and thus deleterious to the accuracy of data acquisition instruments.
Thus, it would be desirable to analyze biochemical and biological systems using assays that employ continuous gradients so as to achieve higher quality data in a shorter time frame and using fewer reagents than present methods. It would further be desirable to utilize continuous concentration gradients within a microfluidic system.